Various methods and apparatus have been known for grinding particulate solids in a liquid media. They include ball mills, pebble mills, roll mills, sand mills, and agitated-media mills. Illustrative art is believed to be U.S. Pat. Nos. 1,577,052, 2,764,359, 2,903,191, 3,008,657, 3,131,875, 3,298,618, 3,149,789 3,204,880, 3,337,140, 3,432,109, 3,591,349, 3,628,965, British Patent Nos. 716,316 and 1,038,153, and German Patent Nos. 1,214,516 and 1,233,237.
The present invention may be performed with various comminuting means such as above mentioned. Typically, however, the invention is performed in agitated-media comminuting apparatus as hereinafter described and claimed wherein the solids in liquid suspension are subjected to generally random contact as well as dynamic screening in a bed of agitated grinding elements. In an agitated-media apparatus, the grinding is usually performed in a vertical cylindrical stationary tank or vessel with a rotatable agitator disposed on a substantially vertical axis. The agitator has one or more solid protuberances such as arms or discs extending out from the axis thereof into a mass of grinding media or elements such as pebbles, or ceramic or metal balls, that occupies a substantial portion of the vessel. The rotation of the protuberances through the mass of grinding media causes the media to occupy an increased apparent volume with the result that the grinding elements have a substantial free space between them and impinge on each other in a manner somewhat similar to the classic model of a gas. The particulate material to be ground and the liquid, which serves as a carrier and dispersing media for the material, occupy the free spaces between the grinding media. The material is ground by the action of the agitated grinding media. A built-in pumping action is usually used to maintain circulation within the comminuting means during grinding.
With respect to the grinding of materials, the present process is applicable to both coarse grinding and fine grinding. It is applicable to the grinding of very hard materials such as iron oxide, of relatively brittle materials such as coal and of relatively soft materials such as clay. The particle size of the solid starting material may vary from particles measuring as little as 325 mesh, or less, to particles measuring as much as 1/4 inch in diameter, or larger. The particle size of the solid starting material is not critical. Further, neither the solid or liquid employed, nor the viscosity of the liquid is critical. However, these factors may require selection as will be apparent to those skilled in the art.
It has been recognized that more mass of material may be ground when the size of the comminuting vessel is increased. Agitated-media apparatus of larger size are, however, relatively more expensive to build and operate. It has also been recognized that the large capital outlay for larger size equipment could be avoided by recycling the suspended solids in the liquid continuum between a comminuting means and a large storage tank, see, e.g., U.S. Pat. No. 3,204,880. Recycling has been, however, generally regarded to result in increased grinding time to reduce the solid particles to a predetermined size. Such recycling operation results in large particles appearing in the final product without having been subjected to a sufficient amount of grinding.
The present invention overcomes these difficulties and disadvantages of the prior art above described. It has been found, contrary to what would be expected, that recycling at relatively high flow rates results in reduced grinding times. It would be expected that the grinding time in a recycling operation would never be as short as the grinding time for the same batch of material ground in a grinding means equal in volume to the retaining tank plush the comminuting means without recirculation, or to a series of grinders equal in total volume to the volume of the retaining tank and comminuting means without recirculation. However, applicant has found that, surprisingly, the grinding time for a given batch reduces the relatively high recirculation rates to less than the time to grind the same batch in a single grinding means, or a series of grinders equal in volume to the retaining tank plus comminuting means. The processing capacity of a given size grinding apparatus is thereby increased without the necessity for relatively large and expensive grinding apparatus. Also, the ground solids produced have been found to have unique properties.